Data Replication Method, Apparatus, and System

ABSTRACT

A data replication method includes obtaining differential data information corresponding to differential data, where the differential data information includes a storage address of the differential data, and a determining value of the differential data, replicating the differential data from the primary volume to the secondary volume according to the storage address of the differential data that is located in the primary volume when the determining value is not less than a preset threshold, and taking a snapshot for the primary volume when the determining value is less than the preset threshold and replicating the differential data to the secondary volume.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 15/405,996filed on Jan. 13, 2017, which is a continuation of Int'l Patent App. No.PCT/CN2014/082229 filed on Jul. 15, 2014, which are incorporated byreference.

FIELD

The present disclosure relates to the field of storage technologies, andin particular, to a data replication method, apparatus, and system.

BACKGROUND

An existing storage system includes a system host that serves as a partythat actually generates data, a primary storage array that stores andcontrols data generated by the system host, and a secondary storagearray that stores and backs up data in the primary storage array. Theprimary storage array includes a primary storage pool that serves as adata storage device and a primary data storage controller that controlsdata stored in the primary storage pool. The primary storage poolincludes a primary volume that is used to receive and store data sent bythe system host and a snapshot volume of the primary volume that is usedto store snapshot data of the primary volume. The secondary storagearray includes a secondary storage pool that serves as a data storagedevice and a secondary data storage controller that controls data storedin the secondary storage pool. The secondary storage pool includes asecondary volume that is used to receive and store backup data of theprimary volume and a snapshot volume of the secondary volume that isused to store snapshot data of the secondary volume. During a process ofgenerating data, data generated by the system host is sent to theprimary volume. The data sent by the system host to the primary volumeis continuously backed up to the secondary volume using a datareplication technology such that data in the secondary volume and thatin the primary volume remains consistent, to achieve a disaster backupeffect when data in the system host is damaged.

In other approaches, data replication is implemented based on a snapshottechnology. A processing process of the snapshot technology is asfollows. A primary data storage controller periodically takes a snapshotfor a primary volume, and replicates, using the snapshot, data sent by asystem host to the primary volume from the primary volume of a primarystorage pool to a secondary volume. In other approaches, a copy-on-write(COW) snapshot or a split-mirror snapshot needs to be taken for theprimary volume if data in the primary volume needs to be modified.During a process of starting a snapshot, performance of the system hostand a primary storage array is greatly affected, and during a process ofsnapshot replication, the data sent by the system host to the primaryvolume cannot be backed up to the secondary volume, thereby increasingan amount of data lost when a disaster occurs.

SUMMARY

Using the present disclosure, the following problems may be resolved. Aproblem in an existing technical solution that performance of a systemhost and a primary storage array is severely affected by frequentlystarting a snapshot, and a problem that during an existing process ofsnapshot replication, an amount of data lost when a disaster occurs islarge because data sent by the system host to a primary volume cannot besynchronized to a secondary volume.

According to a first aspect, an embodiment of the present disclosureprovides a data replication method, including obtaining differentialdata information corresponding to differential data that differentiatesa primary volume from a secondary volume, where the differential datainformation includes a storage address of the differential data that islocated in the primary volume, and a differential data determining valueof the differential data, and the differential data determining valueincludes a data amount of the differential data or a ratio of thedifferential data amount of the differential data to a bandwidth forreplication between the primary volume and the secondary volume,replicating the differential data from the primary volume to thesecondary volume according to the storage address of the differentialdata that is located in the primary volume when the differential datadetermining value is not less than a preset threshold, and taking asnapshot for the primary volume and replicating the differential data tothe secondary volume according to the snapshot when the differentialdata determining value is less than the preset threshold.

With reference to the first aspect, in a first implementation manner ofthe first aspect, replicating the differential data from the primaryvolume to the secondary volume according to the storage address of thedifferential data that is located in the primary volume further includesperforming each round of differential data replication according to asequence of a data replication pointer from a start address in theprimary volume to an end address in the primary volume, and replicatingthe differential data from the primary volume to the secondary volumeaccording to the storage address of the differential data that islocated in the primary volume, where the data replication pointer isused to point to an address of differential data that is currently beingreplicated, and the method includes receiving, by the primary volume,data delivered by a system host during the process of replicating thedifferential data from the primary volume to the secondary volumeaccording to the storage address of the differential data that islocated in the primary volume, replicating the data from the primaryvolume to the secondary volume in a current round of replication if astorage address of the data that is in the primary volume is locatedafter an address currently indicated by the data replication pointer,and skipping replicating the data in the current round of replication ifthe storage address of the data that is in the primary volume is locatedbefore the address currently indicated by the data replication pointer.

According to a second aspect, an embodiment of the present disclosureprovides a primary data storage controller, including a processor, amemory, a communications interface, and a bus, where the processor, thememory, and the communications interface communicate using the bus. Thecommunications interface is configured to communicate with a primarystorage pool and a secondary storage pool, where the primary storagepool includes a primary volume and the secondary storage pool includes asecondary volume. The memory is configured to store a program, and theprocessor is configured to execute the program stored in the memory, toexecute the data replication method provided in the first aspect by anembodiment of the present disclosure or the data replication methodprovided in the first implementation manner of the first aspect by anembodiment of the present disclosure when the primary data storagecontroller runs.

According to a third aspect, an embodiment of the present disclosureprovides a data replication control apparatus, including a differentialdata information acquiring module configured to obtain differential datainformation corresponding to differential data that differentiates aprimary volume from a secondary volume, where the differential datainformation includes a storage address of the differential data that islocated in the primary volume, and a differential data determining valueof the differential data, and the differential data determining valueincludes a data amount of the differential data or a ratio of thedifferential data amount of the differential data to a bandwidth forreplication between the primary volume and the secondary volume, a datareplication module configured to replicate the differential data fromthe primary volume to the secondary volume according to the storageaddress of the differential data that is located in the primary volumewhen the differential data determining value is not less than a presetthreshold, and a snapshot replication module configured to take asnapshot for the primary volume and replicate the differential data tothe secondary volume according to the snapshot when the differentialdata determining value is less than the preset threshold.

According to a fourth aspect, an embodiment of the present disclosureprovides a data replication system, including the primary data storagecontroller, the primary storage pool, and the secondary storage poolthat are provided in the second aspect by an embodiment of the presentdisclosure, where the primary storage pool includes a primary volume andthe secondary storage pool includes a secondary volume.

According to the embodiments provided in the foregoing, during a processin which differential data that differentiates a primary volume from asecondary volume is replicated from the primary volume to the secondaryvolume, no snapshot is taken for the primary volume until a differentialdata determining value decreases to be less than a threshold. Comparedwith an existing technical solution in which a snapshot is frequentlytaken for the primary volume, impact caused by frequently starting asnapshot on performance of a system host and a primary storage array isobviously diminished, and in the embodiments of the present disclosure,during a process of directly replicating the differential data, datasent by the host to the primary volume can be replicated to thesecondary volume, thereby reducing an amount of data lost when adisaster occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

To describe the technical solutions in the embodiments of the presentdisclosure more clearly, the following briefly introduces theaccompanying drawings required for describing the embodiments. Theaccompanying drawings in the following description show merely someembodiments of the present disclosure, and a person of ordinary skill inthe art may still derive other drawings from these accompanying drawingswithout creative efforts.

FIG. 1 is a schematic diagram of an architecture of a data replicationsystem applied in an embodiment of the present disclosure;

FIG. 2 is a schematic flowchart of a data replication method applied inan embodiment of the present disclosure;

FIG. 3A and FIG. 3B are schematic flowcharts of another data replicationmethod applied in an embodiment of the present disclosure;

FIG. 4 is a schematic structural diagram of composition of a deviceapplied in an embodiment of the present disclosure;

FIG. 5 is a schematic structural diagram of composition of an apparatusapplied in an embodiment of the present disclosure; and

FIG. 6 is a schematic structural diagram of composition of a systemapplied in an embodiment of the present disclosure.

DETAILED DESCRIPTION

To make the objectives, technical solutions, and advantages of theembodiments of the present disclosure clearer, the following clearlydescribes the technical solutions in the embodiments of the presentdisclosure with reference to the accompanying drawings in theembodiments of the present disclosure. The described embodiments aresome but not all of the embodiments of the present disclosure. All otherembodiments obtained by a person of ordinary skill in the art based onthe embodiments of the present disclosure without creative efforts shallfall within the protection scope of the present disclosure.

The embodiments of the present disclosure provide a data replicationmethod, apparatus, and system. As shown in FIG. 1, the data replicationsystem includes a system host, a primary storage array, and a secondarystorage array. The system host and the primary storage array establish acommunications connection, and the primary storage array and thesecondary storage array establish a communications connection. Data sentby the system host to the primary storage array is stored in a primaryvolume of a primary storage pool, and backup replication data of data inthe primary volume is stored in a secondary volume of a secondarystorage pool. The primary storage pool and the secondary storage poolare respectively provided with a snapshot volume of the primary volumeand a snapshot volume of the secondary volume, which are respectivelyused to store snapshot data of the primary volume and the secondaryvolume. The primary storage array further includes a primary datastorage controller that controls data stored in the primary storagepool, and the secondary storage array further includes a secondary datastorage controller that controls data stored in the secondary storagepool.

Further, the data sent by the system host to the primary storage arrayis stored in the primary volume, and the primary data storage controllerrecords storage information of the foregoing data that is located in theprimary volume, such as a storage address of the foregoing data that islocated in the primary volume and a data amount of the foregoing data.In addition, the primary data storage controller implements a presetdata replication policy, and replicates the foregoing data from theprimary storage pool to the secondary volume according to the storagedata of the foregoing data that is located in the primary volume, wherethe storage information is recorded in the primary data storagecontroller. During a process of replication, snapshot data that isgenerated due to starting of a snapshot for the primary volume is storedin the snapshot volume of the primary volume. The secondary data storagecontroller records storage information that is in the secondary volumeand is of data replicated from the primary storage pool to the secondaryvolume. During a process in which the secondary volume receives the datathat is replicated from the primary storage pool to the secondaryvolume, snapshot data that is generated by starting a snapshot for thesecondary volume is stored in the snapshot volume of the secondaryvolume.

Based on the foregoing content, the embodiments of the presentdisclosure provide a data replication method, apparatus, and system.

Method Embodiment 1

Method Embodiment 1 of the present disclosure provides a datareplication method, of which a flowchart is shown in FIG. 2. The methodincludes the following steps.

Step 201: Obtain differential data information corresponding todifferential data that differentiates a primary volume from a secondaryvolume, where the differential data information includes a storageaddress of the differential data that is located in the primary volume,and a differential data determining value of the differential data,where the differential data determining value includes a data amount ofthe differential data or a ratio of the differential data amount of thedifferential data to a bandwidth for replication between the primaryvolume and the secondary volume.

A primary data storage controller obtains, according to data informationthat is sent by a system host to the primary volume and recorded by theprimary data storage controller and data information replicated from theprimary volume to the secondary volume, the differential datainformation of the differential data differentiating the primary volumefrom the secondary volume. The foregoing differential data informationincludes the storage address of the differential data that is located inthe primary volume and the differential data determining value of thedifferential data. The foregoing differential data determining valueincludes the data amount of the differential data. In addition, if theprimary data storage controller records the bandwidth for replicationbetween the primary volume and the secondary volume, with reference tothe foregoing data amount of the differential data, time required forreplicating the differential data from the primary volume to thesecondary volume, that is, a ratio of the foregoing data amount of thedifferential data to the bandwidth for replication between the primaryvolume and the secondary volume, may be obtained. The ratio can also beused as the differential data determining value, and the foregoingbandwidth for replication is a communication bandwidth used when data isreplicated between the primary volume and the secondary volume.

Step 202 Replicate the differential data from the primary volume to thesecondary volume according to the storage address of the differentialdata that is located in the primary volume when the differential datadetermining value is not less than a preset threshold.

The preset threshold refers to a threshold corresponding to thedifferential data determining value in step 201. That is, the foregoingpreset threshold is a data amount if the data amount of the differentialdata is selected as the differential data determining value in step 201,or the foregoing preset threshold is a ratio if the ratio of thedifferential data amount of the differential data to the bandwidth forreplication between the primary volume and the secondary volume isselected as the differential data determining value in step 201. Afterthe primary data storage controller determines that the foregoingdifferential data determining value is greater than or equal to theforegoing preset threshold, the differential data is replicated from theprimary volume to the secondary volume according to the storage addressof the differential data that is located in the primary volume. During aprocess of replication, the primary data storage controller records thedifferential data information of the differential data replicated fromthe primary volume to the secondary volume. In addition, the primarydata storage controller also records data information corresponding tothe data sent by the system host to the primary volume if the systemhost sends data to the primary volume during the process of replication.The data information includes a data amount corresponding to the datasent by the system host to the primary volume during the process ofreplication and a storage location that is in the primary volume and isof the data sent by the system host to the primary volume such thatduring the process of replication, the primary data storage controllercan update the differential data information of the differential datadifferentiating the primary volume from the secondary volume.

Step 203 Take a snapshot for the primary volume and replicate thedifferential data to the secondary volume according to the snapshot whenthe differential data determining value is less than the presetthreshold.

After the primary data storage controller determines that the foregoingdifferential data determining value is less than the foregoing presetthreshold, a snapshot is taken for the foregoing primary volume, wherethe foregoing snapshot may be a COW snapshot or a split-mirror snapshot,and the differential data is replicated to secondary volume according tothe snapshot.

A process of taking a COW snapshot for the primary volume is as follows.A mapping table and a resource volume are established in a snapshotvolume of the primary volume when a COW snapshot is established, and apointer recorded in the mapping table points to a data block in theprimary volume, where the pointer recorded in the mapping table includesa pointer that points to the differential data. During existence of theCOW snapshot, original primary volume data stored in storage space thatis allocated to the foregoing data and is in the primary volume isreplicated to the resource volume if the system host sends data to theprimary volume, and then the foregoing data is stored into the storagespace allocated in the primary volume. In addition, a pointer that isrecorded in the mapping table and points to the original primary volumedata stored in the primary volume is changed to point to originalprimary volume data that is replicated to the resource volume forstorage. During the existence of the COW snapshot, regardless of whetherthere is data stored, during the process of replication, into storagespace that stores the differential data and is in the primary volume, itcan be ensured that the pointer that is recorded in the mapping tableand points to the differential data points to differential data to whichthe pointer points when the COW snapshot is established, and thedifferential data is replicated to the secondary volume using thepointer that is recorded in the mapping table and points to thedifferential data. When differential data existing when the COW snapshotis established is all replicated from the primary volume to thesecondary volume, the COW snapshot is canceled.

Taking a split-mirror snapshot for the primary volume is as follows.Primary volume data is replicated to the snapshot volume of the primaryvolume when a split-mirror snapshot is established. Then thedifferential data that differentiates the primary volume from thesecondary volume is converted into differential data that differentiatesthe snapshot volume of the primary volume from the secondary volume, andthe differential data that differentiates the snapshot volume of theprimary volume from the secondary volume is replicated from the snapshotvolume of the primary volume to the secondary volume. During existenceof the split-mirror snapshot, the foregoing data is directly stored inthe primary volume if the system host sends data to the primary volume,and the split-mirror snapshot is canceled when differential dataexisting when the split-mirror snapshot is established is all replicatedfrom the snapshot volume of the primary volume to the secondary volume.

In this method embodiment, a primary data storage controller directlyreplicates differential data from a primary volume to a secondary volumewhen a differential data determining value is greater than or equal to apreset threshold, thereby greatly reducing a quantity of times ofstarting a snapshot for the primary volume, and diminishing impactcaused by frequently starting a snapshot on performance of a system hostand a primary storage array.

Method Embodiment 2

In Method Embodiment 2 of the present disclosure, based on MethodEmbodiment 1 of the present disclosure, before the differential datadetermining value is compared with the preset threshold, that is, beforeit is determined in step 202 that the differential data determiningvalue is greater than or equal to the preset threshold or it isdetermined in step 203 that the differential data determining value isless than the preset threshold, a snapshot is taken for the secondaryvolume, where the foregoing snapshot may be a COW snapshot or asplit-mirror snapshot. After differential data existing when thesnapshot for the primary volume is started is all replicated to thesecondary volume, that is, after the differential data is replicated tothe secondary volume according to the snapshot in step 203, a snapshotfor the secondary volume is canceled.

In this method embodiment, before differential data is replicated from aprimary volume to a secondary volume, a snapshot is taken for thesecondary volume, and a snapshot for the secondary volume is canceledafter the differential data existing when a snapshot for the primaryvolume is started is all replicated to the secondary volume, therebyensuring that during a process in which the secondary volume receivesthe differential data, data in the secondary volume may be rolled backto a state at a moment when the snapshot for the secondary volume isstarted.

Method Embodiment 3

In Method Embodiment 3 of the present disclosure, based on MethodEmbodiment 1, replicating the differential data from the primary volumeto the secondary volume according to the storage address of thedifferential data that is located in the primary volume in step 202 inMethod Embodiment 1 further includes performing each round ofdifferential data replication according to a sequence in which a datareplication pointer points to from a start address in the primary volumeto an end address in the primary volume. As shown in FIG. 3A, the datareplication pointer runs according to a sequence from A1 to A5 andpoints to an address of differential data currently being replicated,and the differential data is replicated from the primary volume to thesecondary volume according to a storage address of the differential datathat is located in the primary volume.

Within a period when the differential data is replicated from theprimary volume to the secondary volume, the system host sends data tothe primary volume. If a storage address of the data sent by the systemhost that is located in the primary volume is located after an addressto which the data replication pointer currently points, that is, asshown in FIG. 3A, the data sent by the system host is stored in alocation of A4 in the primary volume and the data replication pointercurrently points to a location of A3, the foregoing data sent by thesystem host is replicated in a current round of replication. If thestorage address of the data sent by the system host that is located inthe primary volume is located before the address to which the datareplication pointer currently points, that is, as shown in FIG. 3B, thedata sent by the system host is stored in a location of A2 in theprimary volume and the data replication pointer currently points to thelocation of A3, the foregoing data sent by the system host is notreplicated in the current round of replication. After being stored atthe location of A2, the foregoing data sent by the system host isreplicated to the secondary volume during a process of a next round ofdifferential data replication.

Device Embodiment 1 of the present disclosure provides a primary datastorage controller, and as shown in FIG. 4, the primary data storagecontroller at least includes a processor 401, a memory 402, acommunications interface 403, and a bus 404, where the processor 401,the memory 402, and the communications interface 403 communicate usingthe bus 404.

The memory 402 is configured to store a program. Further, the programmay include program code, where the program code includes a computeroperation instruction. The memory 402 may be a high-speed random accessmemory (RAM) memory, or may be a non-volatile memory, such as at leastone disk storage.

The processor 401 is configured to execute the operation instructionstored in the memory 402, and may be a single-core or a multi-corecentral processing unit (CPU), an application specific integratedcircuit (ASIC), or one or more integrated circuits configured toimplement this embodiment of the present disclosure.

The communications interface 403 is configured to communicate with aprimary storage pool and a secondary storage pool, where the primarystorage pool includes a primary volume and the secondary storage poolincludes a secondary volume.

The processor 401 runs the program, to execute any one of the foregoingMethod Embodiment 1, Method Embodiment 2, and Method Embodiment 3 whenthe controller runs.

Corresponding to the foregoing method embodiments of the presentdisclosure, Apparatus Embodiment 2 of the present disclosure provides adata replication control apparatus, and as shown in FIG. 5, theapparatus includes a differential data information acquiring module 501configured to obtain differential data information corresponding todifferential data that differentiates a primary volume from a secondaryvolume, where the differential data information includes a storageaddress of the differential data that is located in the primary volume,and a differential data determining value of the differential data,where the differential data determining value includes a data amount ofthe differential data or a ratio of the data amount of the differentialdata to a bandwidth for replication between the primary volume and thesecondary volume, a data replication module 502 configured to replicatethe differential data from the primary volume to the secondary volumeaccording to the storage address of the differential data that islocated in the primary volume when the differential data determiningvalue is not less than a preset threshold, and a snapshot replicationmodule 503 configured to take a snapshot for the primary volume andreplicate the differential data to the secondary volume using thesnapshot when the differential data determining value is less than thepreset threshold.

The present disclosure further provides an embodiment of a datareplication system as shown in FIG. 6, and the system includes a primarydata storage controller 601, a primary storage pool 602, and a secondarystorage pool 603 that are provided in the foregoing apparatusembodiment, where the primary storage pool 602 includes a primary volume6021 and the secondary storage pool 603 includes a secondary volume6031. The primary data storage controller 601 establishes communicationwith the primary storage pool 602, and the primary storage pool 602establishes communication with the secondary storage pool 603.

An embodiment of the present disclosure further provides acomputer-readable medium, including a computer operation instruction,where the computer operation instruction is configured to execute themethods described in any one of method embodiments of the presentdisclosure by a controller.

It should be noted that, for ease of description, the foregoing methodembodiments are described as a series of action combinations. However, aperson skilled in the art should understand that the present disclosureis not limited to the described sequence of the actions, because somesteps may be performed in another sequence or performed at the same timeaccording to the present disclosure. In addition, a person skilled inthe art should also understand that all the embodiments described inthis specification belong to exemplary embodiments, and the involvedactions and modules are not necessarily mandatory to the presentdisclosure.

In the foregoing embodiments, the description of each embodiment hasrespective focuses. For a part that is not described in detail in anembodiment, reference may be made to related descriptions in otherembodiments.

With descriptions of the foregoing embodiments, a person skilled in theart may clearly understand that the present disclosure may beimplemented by hardware, firmware or a combination thereof. When thepresent disclosure is implemented by software, the foregoing functionsmay be stored in a computer-readable medium or transmitted as one ormore instructions or code in the computer-readable medium. Thecomputer-readable medium includes a computer storage medium and acommunications medium, where the communications medium includes anymedium that enables a computer program to be transmitted from one placeto another. The storage medium may be any available medium accessible toa computer. The following provides an example but does not impose alimitation. The computer-readable medium may include a RAM, a read-onlymemory (ROM), an electrically erasable programmable ROM (EEPROM), acompact disc-ROM (CD-ROM), or another optical disc storage or a diskstorage medium, or another magnetic storage device, or any other mediumthat can carry or store expected program code in a form of aninstruction or a data structure and can be accessed by a computer. Inaddition, any connection may be appropriately defined as acomputer-readable medium. For example, if software is transmitted from aweb site, a server or another remote source using a coaxial cable, anoptical fiber/cable, a twisted pair, a digital subscriber line (DSL) orwireless technologies such as infrared ray, radio and microwave, thecoaxial cable, optical fiber/cable, twisted pair, DSL or wirelesstechnologies such as infrared ray, radio and microwave are included infixation of a medium to which they belong. For example, a disk and adisc used by the present disclosure includes a compact disc (CD), alaser disc, an optical disc, a digital versatile disc (DVD), a floppydisk and a BLU-RAY DISC, where the disk generally copies data by amagnetic means, and the disc copies data optically by a laser means. Theforegoing combination should also be included in the protection scope ofthe computer-readable medium.

Finally, it should be noted that the foregoing embodiments are merelyintended for describing the technical solutions of the presentdisclosure but not for limiting the present disclosure. Although thepresent disclosure is described in detail with reference to theforegoing embodiments, persons of ordinary skill in the art shouldunderstand that they may still make modifications to the technicalsolutions described in the foregoing embodiments or make equivalentreplacements to some technical features thereof, without departing fromthe spirit and scope of the technical solutions of the embodiments ofthe present disclosure.

What is claimed is:
 1. A data replication method comprising: obtainingdifferential data information corresponding to differential data thatdifferentiate a primary device from a secondary device, wherein thedifferential data information comprises a storage address of thedifferential data that are located in the primary device and adetermining value of the differential data, where the determining valuedetermines when to initiate a snapshot; taking a primary snapshot forthe primary device when the determining value is less than a presetthreshold; and replicating the differential data to a secondary volumeaccording to the primary snapshot when the determining value is lessthan the preset threshold.
 2. The data replication method of claim 1,wherein the determining value comprises a ratio of a data amount of thedifferential data to a bandwidth for replication between a primaryvolume and the secondary volume.
 3. The data replication method of claim1, wherein the determining value comprises a data amount of thedifferential data.
 4. The data replication method of claim 1, furthercomprising: generating first snapshot data due to starting of theprimary snapshot; and storing, during the replicating, the firstsnapshot data in a primary snapshot volume of a primary volume.
 5. Thedata replication method of claim 4, further comprising: generatingsecond snapshot data by starting a secondary snapshot for the secondarydevice; and storing, while the secondary device receives thedifferential data, the second snapshot data in a secondary snapshotvolume of the secondary device.
 6. The data replication method of claim1, wherein the snapshot is a copy-on-write (COW) snapshot or asplit-mirror snapshot.
 7. The data replication method of claim 1,wherein before the replicating, the data replication method furthercomprises: taking a secondary snapshot for the secondary device; andcancelling the secondary snapshot after the differential data existingwhen the primary snapshot is started is all replicated to the secondarydevice.
 8. A primary data storage controller comprising: a memoryconfigured to store a program; and a processor coupled to the memory andconfigured to: obtain differential data information corresponding todifferential data that differentiate a primary device from a secondarydevice, wherein the differential data information comprises a storageaddress of the differential data that are located in the primary deviceand a determining value of the differential data, wherein thedetermining value determines when to initiate a snapshot; take a primarysnapshot for the primary device when the determining value is less thana preset threshold; and replicate the differential data to a secondaryvolume according to the primary snapshot when the determining value isless than the preset threshold.
 9. The primary data storage controllerof claim 8, wherein the determining value comprises a ratio of a dataamount of the differential data to a bandwidth for replication between aprimary volume and the secondary volume.
 10. The primary data storagecontroller of claim 8, wherein the determining value comprises a dataamount of the differential data.
 11. The primary data storage controllerof claim 8, wherein the processor is further configured to: generatefirst snapshot data due to starting of the primary snapshot; and store,during the replicating, the first snapshot data in a primary snapshotvolume of a primary volume.
 12. The primary data storage controller ofclaim 11, wherein the processor is further configured to: generatesecond snapshot data by starting a secondary snapshot for the secondarydevice; and store, while the secondary device receives the differentialdata, the second snapshot data in a secondary snapshot volume of thesecondary device.
 13. The primary data storage controller of claim 8,wherein the snapshot is a copy-on-write (COW) snapshot or a split-mirrorsnapshot.
 14. The primary data storage controller of claim 8, whereinbefore the replicating, the processor is further configured to: take asecondary snapshot for the secondary device; and cancel the secondarysnapshot after the differential data existing when the primary snapshotis started is all replicated to the secondary device.
 15. A computerprogram product comprising instructions that are stored on acomputer-readable medium and that, when executed by a processor, cause aprimary data storage controller to: obtain differential data informationcorresponding to differential data that differentiate a primary devicefrom a secondary device, wherein the differential data informationcomprises a storage address of the differential data that are located inthe primary device and a determining value of the differential data,wherein the determining value determines when to initiate a snapshot;take a primary snapshot for the primary device when the determiningvalue is less than a preset threshold; and replicate the differentialdata to a secondary volume according to the primary snapshot when thedetermining value is less than the preset threshold.
 16. The computerprogram product of claim 15, wherein the determining value comprises aratio of a data amount of the differential data to a bandwidth forreplication between a primary volume and the secondary volume.
 17. Thecomputer program product of claim 15, wherein the determining valuecomprises a data amount of the differential data.
 18. The computerprogram product of claim 15, wherein when executed by the processor, theinstructions further cause the primary data storage controller to:generate first snapshot data due to starting of the primary snapshot;store, during the replicating, the first snapshot data in a primarysnapshot volume of a primary volume; generate second snapshot data bystarting a secondary snapshot for the secondary device; and store, whilethe secondary device receives the differential data, the second snapshotdata in a secondary snapshot volume of the secondary device.
 19. Thecomputer program product of claim 15, wherein the snapshot is acopy-on-write (COW) snapshot or a split-mirror snapshot.
 20. Thecomputer program product of claim 15, wherein before the replicating andwhen executed by the processor, the instructions further cause theprimary data storage controller to: take a secondary snapshot for thesecondary device; and cancel the secondary snapshot after thedifferential data existing when the primary snapshot is started is allreplicated to the secondary device.